In a hot runner mold, it is necessary to maintain material within the runner in a molten condition by heating the runner. Heater heating and induction heating have been known as methods for heating the runner. The induction heating can heat the material to a desired temperature in a shorter time than the heater heating. Further, the induction heating has the advantage that it is superior in controllability of temperature and power consumption is also small.
FIG. 10 is for showing a prior art of the present invention and is a schematic side view of a manifold heated by induction heating.
A manifold 10 has a sprue portion 11 for introducing molten material sent from an injection cylinder (not shown) into the manifold 10 and a runner portion 12 for distributing the molten material as flown from the sprue portion 11 to nozzles 20. The manifold 10 is disposed within a space formed between a fixed mold plate 1 of the hot runner mold and a back plate 2 mounted to a rear surface thereof. At a time of injection molding, a large mold clamping force acts on the fixed mold plate 1, the nozzles 20 and the back plate 2. For this reason, the manifold 10 is supported by heat-insulating supporting members 16 at a plurality of portions between the fixed mold plate 1 and the back plate 2 so as to prevent the manifold 10 from being distorted or displaced due to the clamping force.
Coils 15 for induction heating are wound around an outer periphery of the manifold 10 at portions at which the manifold is not supported by the supporting members 16. When voltage is applied to the induction heating coils 15, the material within the runner is heated at portions where the induction heating coils 15 are wound.
In the above-described induction heating method there are the following drawbacks.
(1) In order to heat material uniformly, it is preferable to wind the coils 15 over the entire length of the runner portion 12. However, portions where the coils 15 may be wound will be limited by the supporting members 16 for supporting the manifold 10. Therefore, there is a drawback that variations occur in temperature of the molten material within the runner to result in irregularities in qualities of injection-molded articles and in pouring deficiencies.
(2) In case of injection molding of molten metal having a high melting point and exhibiting favorable thermal conductivity, such as magnesium, it will be required to heat the material located at portions where induction heating is performed to a temperature that is remarkably higher than the melting point thereof. This consequently increases the possibility of run-out or thermal distortion and also shortens the life of the coils themselves.
(3) As one method for solving the above drawback, it is considered to form through holes on the supporting members and to wind the induction heating coils around the periphery of the manifold 10 through these through holes. However, forming the through holes into the supporting members formed of material such as ceramics will result in higher cost. The through holes will further weaken the strength of the supporting members.
(4) The respective coils 15 are connected by lead wires each other. These lead wires are connected to an external power supply source through the back plate 2. A drawback is accordingly presented that the coils 15 are hard to be detached from the manifold 10 at a time of performing exchange, inspection or repair of the manifold 10 and thus worsens the workability.
The present invention has been made in view of these drawbacks, and it is an object thereof to provide an induction heating method for a manifold of a hot runner mold and a coil unit for induction heating with which it is possible to uniformly heat material over the entire length of a runner without weakening the strength of a supporting member and in which the coil may be easily attached to and detached from the manifold.